The treads of modern tires must meet performance standards which require a broad range of desirable properties. Thus, rubber compositions suitable for tire treads should exhibit not only desirable strength and elongation, particularly at high temperatures, but also good cracking resistance, good abrasion resistance, desirable skid resistance and low tan delta values at low frequencies for desirable rolling resistance. Additionally, a high complex dynamic modulus is necessary for maneuverability and steering control.
It has been recognized that this complex balance of properties, required in the modern tire, cannot be satisfied by a single polymer and that, therefore, a blend of polymers must be employed. See, e.g., J. Walker et al, "Specialty Elastomers in Tires", Elastomerics, September 1985, pp. 22-29. It is noteworthy that all of the polymers employed in the blends described in such article possess a high degree of unsaturation. Consequently, such blends are readily vulcanized by the use of accelerated sulfur curative systems.
Ethylene/alphaolefin/nonconjugated diene terpolymers ("EPDM") are well known to have a significant advantage in their resistance to aging and to oxidation, because of their low degree of unsaturation and the position of the unsaturation which is not in the polymer backbone. Moreover, other advantages of using EPDM in treads include a high dynamic modulus for steering control and a low tan delta value at low frequencies for desirable rolling resistance.
However, it is well established that tire treads employing EPDM as the sole polymer are deficient in skid resistance. See, e.g., U.S. Pat. No. 4,195,013 to de Zarauz. Moreover, due to the low unsaturation of EPDM, it is difficult to adhere such a tread to tire carcasses composed of unsaturated rubbers.
In the past, covulcanizates of EPDM and highly unsaturated rubbers have generally exhibited poor strength and high hysteresis when cured with conventional accelerated sulfur vulcanization systems. Accordingly, several different approaches have been undertaken to find a method of achieving a desirable covulcanization of blends of rubber of varying degrees of saturation, particularly blends of highly unsaturated rubbers and EPDM.
One frequently employed approach has been to halogenate the highly saturated rubber (typically "EPDM") thereby rendering a blend of such halogen-modified rubber with an unsaturated rubber sulfur curable. Thus, Schoen et al (U.S. Pat. No. 3,936,430) shows sulfur-curable blends of halogenated EPDM with unsaturated rubbers. (It is noteworthy that Schoen et al indicate at Column 4, lines 45-46 that the use of peroxide is "unnecessary".) Similarly, Hopper (U.S. Pat. No. 4,017,468) shows sulfur-curable blends of N-chlorothiocarboxylic amides or imides with unsaturated rubbers, whereas Kresge et al (U.S. Pat. No. 3,524,826) discloses sulfur-cured covulcanizates of brominated EPDM and highly unsaturated rubbers. In the same vein, Landi et al (U.S. Pat. No. 3,956,247) show a sulfur-curable blend of halogenated EPDM and diene rubber, and Hopper (in U.S. Pat. No. 3,915,907) shows sulfur curable blends of N-chlorothiosulfonamide-modified EPDM and diene rubbers.
Somewhat similarly, Son et al (in U.S. Pat. No. 3,897,405) show blends of (i) EPDM having --SH or --S--N linkages grafted thereto, with (ii) highly unsaturated rubbers, which blends are sulfur curable.
A second general approach taken to achieve the cocure of otherwise incompatible rubbers involves the use of a particular sulfur-cure accelerator or combination of sulfur-cure accelerators in conjunction with the use of sulfur. Thus, Mastromatteo et al (in U.S. Pat. No. 3,678,135) show the use of long chain hydrocarbon dithiocarbamate accelerators, such as zinc N-dodecyl, N-isopropyl dithiocarbamate in the sulfur cure of EPDM with highly unsaturated rubbers, while, similarly, Taylor et al (in U.S. Pat. No. 4,008,190) show the sulfur cure of EPDM with highly unsaturated diene rubbers employing a N,N-(higher alkyl)thiocarbamylsulfenamide as a vulcanization accelerator. In a similar manner, Hines et al (in U.S. Pat. No. 3,343,582) disclose a vulcanizable composition comprised of polyisoprene and EPDM, which composition contains as cure accelerators a benzothiazyl sulfide compound, an aryl-substituted guanidine and a thiuram sulfide.
Other approaches taken in the past to achieve such a cocure include that of Knabeschuh et al (in U.S. Pat. No. 3,817,952) which involves preparing a branched EPDM by heating EPDM with sulfur or a peroxide, and then blending such branched EPDM with a diene to form a sulfur or peroxide curable composition; that of Marchesini et al (U.S. Pat. No. 3,299,175) which employs the polymerization product of divinyl benzene and a vinyl unsaturated polymer as a filler for a peroxide-curable ethylene/alphaolefin copolymer (although Marchesini does state that sulfur can be employed in addition to peroxide, all Examples show peroxide only); and that of Wei et al (U.S. Pat. No. 3,179,718) which shows a vulcanized blend of nitrile rubber and ethylene/propylene copolymer which blend has been vulcanized employing both an organic peroxide and elemental sulfur. (It is noteworthy that Wei et al mentions neither EPDM nor sulfur cure accelerators.)
Both Bohm et al (U.S. Pat. No. 4,350,795) and Petersen (U.S. Pat. No. 4,202,801) show thermoplastic elastomers comprised of blends of a polyalphaolefin, EPDM and a conjugated diene rubber. While both these patents state that combination of sulfur and peroxide may be employed, Bohm et al show the use of sulfur only while Petersen exemplifies the use of peroxide only. Moreover, it is readily apparent that thermoplastic elastomers, due to their thermoplastic nature, are unsuitable for use as tire tread compositions.
Thus, it would be desirable to possess a tire having a tread composition which exhibits desirable strength and elongation as well as desirable rolling resistance, and anti-skid and tractive properties.
Accordingly, it is an object of this invention to provide a tire having a tread composition which exhibits desirable strength and elongation as well as desirable rolling resistance, anti-skid and tractive properties.
It is a further object of this invention to provide a tire having a tread composition comprised of a covulcanizate of a highly unsaturated and saturated rubber.
These objects, and other additional objects will become more apparent from the following description and accompanying Examples.